The present invention concerns the problem of improving the performance of a resonant circuit and, more particularly, the startup of a resonant tuning fork driven by an amplifier in a phase locked loop circuit.
Piezoelectric vibratory gyroscopes utilizing vibratory eletrostatically driven tuning forks are being used in a variety of angular velocity measuring applications. In such applications the tuning fork may be excited in a closed loop drive circuit which includes a phase locked loop.
Phase locked loop resonant drive circuits measure the phase difference between a sensor signal indicative of vibration of tuning fork and the drive signal applied to the amplifier providing the drive signal which drives the vibration of the tuning fork. A problem may arise in the starting of such circuits because upon the initial application of power, the tuning fork produces no output feedback signal to the phase comparator because it is not yet vibrating. At that time the output frequency of the voltage contolled oscillator, VCO, has no correlation to the tuning fork frequency because it is initially receiving a zero input signal. Such systems start up, eventually, when amplified white noise present in the system starts to be positively fed back through the system.
Where the tuning fork sensor has a resonant frequency f0, and the efficiency of the resonant system is Q, the starting time may be determined by dividing Q divided by f0. For vacuum tuning fork sensors with electrostatic drive and pickoffs, a typical Q of 100,000 and f0 of 20,000 may require starting times of as long as five seconds. While this starting time could be reduced if the tuning fork were initially overdriven at the resonant frequency at the time that it was turned on, it has proven difficult to predict the resonant frequency of the fork in a particular circuit since it is common for them to have a resonant frequency that may vary in the xc2x120% range between similar tuning forks. Also, to be effective, the drive frequency must be within xc2x1f0 divided by Q cycles of the sensor resonant frequency.
A phase locked loop circuit for driving an oscillatory mechanical object, which, when constructed according to the preferred embodiments of the present invention uses an amplifier for providing a drive signal to the mechanical object in response to an input signal, a phase detector for receiving a first signal and a second signal proportional to the movement of the mechanical object, the phase detector constructed and arranged for comparing the phase of the first and second signals and for providing an output signal having an average voltage proportional to the phase difference between the first and second signals, a voltage controlled oscillator receiving the output signal from the phase detector and producing an output signal which is the first signal received by the phase detector and an input of the amplifier, and a signal source for providing the voltage controlled oscillator with a starting signal at the time that power is applied to the system such that the frequency of the output of the voltage controlled oscillator commences at an initial frequency of the voltage controlled oscillator which is higher than a resonant frequency of the mechanical object, the signal source constructed and arranged for reducing the output frequency of the voltage controlled oscillator until the it corresponds to the resonant frequency of the mechanical object.
The apparatus and method may also use a voltage controlled oscillator, a driver circuit for providing a signal for driving the mechanical object at a frequency determined by the voltage controlled oscillator, a drive signal source for providing a starting drive signal to the voltage controlled oscillator when power is applied to the circuit prior to commencement of the oscillation of the mechanical object, the starting drive signal providing an output from the voltage controlled oscillator at a starting frequency predetermined to be above the range of expected resonant frequencies of the mechanical object, the starting drive signal varying with time to reduce the output frequency of the voltage controlled oscillator, a phase detector for comparing the phase difference between the output of the voltage controlled oscillator and a feedback signal indicative of the oscillatory motion of the mechanical object and providing an error signal to the input of the voltage controlled oscillator having an average voltage proportional to that phase difference, and a switch for removing the starting drive signal from the input of the voltage controlled oscillator when the frequency of the oscillatory motion of the mechanical object reaches a resonant frequency of the mechanical object.
A method of starting a closed loop resonant drive circuit for a mechanical object may involve providing an open loop starting signal to the circuit for initially driving the mechanical object at a frequency well above the range of frequencies expected for the resonant frequency of the mechanical object, varying the starting signal for sweeping the driving frequency to lower frequencies until the resonant frequency of the mechanical object is reached, and removing the starting signal while the mechanical object continues to oscillate at the resonant frequency.
A method of rapidly initiating oscillation of a resonant mechanical system driven by a phase lock loop circuit comprising an amplifier having a positive feedback path may also involve applying a decaying open loop starting voltage to the VCO (voltage controlled oscillator) input of a phase lock loop circuit so that the phase lock loop circuit output frequency starts at a maximum frequency which is above the resonant frequency of the mechanical system and sweeps downwardly toward a minimum frequency below the resonant frequency of mechanical system, allowing the phase locked loop circuit to lock at the resonant frequency of the mechanical element when the frequency of VCO reaches the resonant frequency of the mechanical system; and removing the starting voltage.
A method of rapidly initiating oscillation of a resonant mechanical system driven by a phase lock loop circuit comprising an amplifier having a positive feedback path may involve applying a decaying open loop starting voltage to the voltage controlled oscillator input of a phase lock loop circuit so that the phase lock loop circuit output frequency starts at a maximum frequency which is above the resonant frequency of the mechanical system and sweeps downwardly toward a minimum frequency below the resonant frequency of mechanical system, allowing the phase locked loop circuit to lock at the resonant frequency of the mechanical element when the frequency of the voltage controlled oscillator reaches the resonant frequency of the mechanical element, generating a lock signal to indicate that the phase locked loop circuit is locked, and removing the starting voltage in response to the lock signal.